Structural, Vibrational, and Electronic Characteristics of Enyne

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J. Am. Chem. Soc. 2000, 122, 6917-6928

6917

Structural, Vibrational, and Electronic Characteristics of Enyne Macrocycles as a Function of Ring Strain Sara Eisler, Robert McDonald, Glen R. Loppnow, and Rik R. Tykwinski* Contribution from the Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, T6G 2G2 Canada ReceiVed February 21, 2000

Abstract: The cross-conjugated macrocycles 8a-e and acyclic model compound 12 have been synthesized and thoroughly analyzed by 1H and 13C NMR, UV, IR, and Raman spectroscopies, and by X-ray crystallography. The increasing ring strain and resultant changes in the double and triple bond orders are clearly detailed in the Raman and 13C NMR spectra, as well as in the X-ray structures. Correlations between 13C NMR shifts, bond angles, and Raman frequencies are essentially linear for the butadiynyl and olefinic moieties. The most highly strained system, 8a, displays interior alkylidene bond angles that are reduced to 108° and alkyne angles reduced to as little as 155°. The electronic absorption spectrum of 8a shows evidence of through-space homoconjugation that is manifested as a bathochromic shift of the absorption band arising from the in-plane π-system. The absorption bands from the out-of-plane π-electron systems of the more strained cycles 8a and 8b show slight bathochromic shifts as compared with the less strained systems 8c-e and acyclic 12. Ring strain, however, has a surprisingly small effect on the electronic absorption energies of these macrocycles.

Introduction The sp-hybridization of carbon in an alkyne moiety dictates a linear geometry for the CsCtC bond. The fact that this angle is considerably more flexible than either a CsCdC or CsCs C bond allows for fairly large deviations from 180° to occur without significant changes in energy.1 As a result, Krebs has gone so far as to specify that only compounds in which the CsCtC bond angle has been reduced to less than 170° are even considered strained.1 The first strained cycloalkynes, cyclononyne2 and cyclooctyne,3 reported in the early 1950s by Blomquist, ushered in an era of fascination with the synthesis and study of cyclic alkynes and enynes. Almost five decades later, the aesthetically pleasing structures and interesting chemical and biological4 reactivity of these macrocycles continue more than ever to capture the imagination of chemists.1,5-17 * To whom correspondence should be addressed. E-mail: [email protected]. (1) Krebs, A.; Wilke, J. Top. Curr. Chem. 1983, 109, 189-233. (2) Blomquist, A. T.; Liu, L. H.; Bohrer, J. C. J. Am. Chem. Soc. 1952, 74, 3643-3647. (3) Blomquist, A. T.; Liu, L. H. J. Am. Chem. Soc. 1953, 75, 21532154. (4) Nicolaou, K. C.; Smith, A. L. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH: Weinheim, 1995; pp 203-283. (5) Modern Acetylene Chemistry; Stang, P. J., Deiderich, F., Eds.; VCH: Weinheim, 1995. (6) Carbon Rich Compounds II. Top. Curr. Chem. 1999, 201 (special issue). (7) Moore, J. S. Acc. Chem. Res. 1997, 30, 402-413. (8) (a) Tykwinski, R. R.; Diederich, F. Liebigs Ann./Recl. 1997, 649661. (b) Diederich, F. Nature (London) 1994, 369, 199-207. (c) Diederich, F.; Rubin, Y. Angew. Chem., Int. Ed. Engl. 1992, 31, 1101-1123. (9) Sander, W. Angew. Chem., Int. Ed. Engl. 1994, 33, 1455-1456. (10) (a) Gleiter, R.; Scha¨fer, W. In The Chemistry of Triple-Bonded Functional Groups: Supplement C2; Patai, S., Ed.; John Wiley & Sons: Chichester, 1994; Vol. 2, pp 153-189. (b) Gleiter, R. Angew. Chem., Int. Ed. Engl. 1992, 31, 27-44. (11) Nakagawa, M. In The Chemistry of the Carbon-Carbon Triple Bond; Patai, S., Ed.; John Wiley & Sons: Chichester, 1978; pp 635-712. (12) Haley, M. M. Synlett 1998, 557-565. (13) Scott, L. T. Pure Appl. Chem. 1986, 58, 105-110.

Cyclic enynes are a particularly interesting category of strained alkynes.18 Included in this category are dehydroannulenes and dehydrobenzoannulenes (DBAs), molecules that have been widely studied since Sondheimer’s seminal work almost 30 years ago.12,14,19 Within the large range of known DBAs are examples of the most strained alkynes to be characterized, some with bond angles of less than 155°, i.e., 1.20 Recently, the ring strain in DBAs such as 2 has been harnessed for the formation of polydiacetylene polymers and carbon nanostructures via thermal topochemical polymerization.21-24 Enyne macrocycles and related cage compounds play a key role in current synthetic endeavors toward C60 and other fullerenes.17,25-30 Typically, these approaches utilize suitably functionalized precursors to afford cyclic polyynes that could enter into a polyyne cyclization mechanism, ultimately expected to yield a fullerene product. It is assumed that this reaction (14) Hoffmann, R. W. Dehydrobenzene and Cycloalkynes; Academic Press: New York, 1967. (15) Sakurai, H.; Nakadaira, Y.; Hosomi, A.; Eriyama, Y.; Kabuto, C. J. Am. Chem. Soc. 1983, 105, 3359-3360. (16) Rubin, Y. Chem. Eur. J. 1997, 3, 1009-1016. (17) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 107119. (18) (a) Meier, H.; Hanold, N.; Molz, T.; Bissinger, H. J.; Kolshorn, H.; Zountsas, J. Tetrahedron 1986, 42, 1711-1719. (b) Meier, H.; Petersen, H.; Kolshorn, H. Chem. Ber. 1980, 113, 2398-2409. (19) Sondheimer, F. Acc. Chem. Res. 1972, 5, 81-91. (20) de Graaff, R. A. G.; Gorter, S.; Romers, C.; Wong, H. N. C.; Sondheimer, F. J. Chem. Soc., Perkin Trans. 2 1981, 478-480. (21) Baldwin, K. P.; Matzger, A. J.; Scheiman, D. A.; Tessier, C. A.; Vollhardt, K. P. C.; Youngs, W. J. Synlett 1995, 1215-1218. (22) Zhou, Q.; Carroll, P. J.; Swager, T. M. J. Org. Chem. 1994, 59, 1294-1301. (23) Boese, R.; Matzger, A. J.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1997, 119, 2052-2053. (24) Dosa, P. I.; Erben, C.; Iyer, V. S.; Vollhardt, K. P. C.; Wasser, I. M. J. Am. Chem. Soc. 1999, 121, 10430-10431. (25) Kiang, C.-H.; Goddard,W. A., III. Phys. ReV. Lett. 1996, 76, 25152518. (26) Rubin, Y.; Parker, T. C.; Pastor, S. J.; Jalisatgi, S.; Boulle, C.; Wilkins, C. L. Angew. Chem., Int. Ed. 1998, 37, 1226-1229.

10.1021/ja000617g CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000

6918 J. Am. Chem. Soc., Vol. 122, No. 29, 2000

Eisler et al. Scheme 1. Synthesis of Macrocycles 8a-e

sequence is initiated as a result of the reactivity and ring strain of the polyyne intermediate.8b,c,31 In the above examples and in the majority of reports concerning strained cyclic acetylenes, it has been the stability and/or reactivity as a function of ring strain that has been examined. Comparatively, the electronic aspects of strained alkynes have been much less studied.32,33 For nonconjugated systems, extensive studies by Gleiter and co-workers have determined the electronic effects of ring strain for isolated ethynes and butadiynes.10,34 Scott, de Meijere, and co-workers have reported on the electronic effects of homoconjugation in a range of macrocyclic polyacetylenes.6,13,35 Neither these studies, nor others,11,36,37 however, have provided significant insight into the structural and electronic attributes of cyclic alkynes with extended conjugation. As the role of alkynes in (27) Tobe, Y.; Fujii, T.; Matsumoto, H.; Tsumuraya, K.; Noguchi, D.; Nakagawa, N.; Sonoda, M.; Naemura, K.; Achiba, Y.; Wakabayashi, T. J. Am. Chem. Soc. 2000, 122, 1762-1775. Tobe, Y.; Nakanishi, H.; Sonoda, M.; Wakabayashi, T.; Achiba, Y. Chem. Commun. 1999, 1625-1626. Tobe, Y.; Nakagawa, N.; Naemura, K.; Wakabayashi, T.; Shida, T.; Achiba, Y. J. Am. Chem. Soc. 1998, 120, 4544-4545. (28) Bunz, U. H. F.; Roidl, G.; Altmann, M.; Enkelmann, V.; Shimizu, K. D. J. Am. Chem. Soc. 1999, 121, 10719-10726. (29) Tykwinski, R. R.; Diederich, F.; Gramlich, F.; Seiler, P. HelV. Chim. Acta 1996, 79, 634-645. (30) Fallis, A. G. Can. J. Chem. 1999, 77, 159-177. (31) Goroff, N. S. Acc. Chem. Res. 1996, 29, 77-83. (32) Meier, H. AdV. Strain Org. Chem. 1991, 1, 215-272. (33) Misumi, S.; Kaneda, T. In The Chemistry of the Carbon-Carbon Triple Bond Part 2; Patai, S., Ed.; John Wiley & Sons: Chichester, 1978; pp 713-737. (34) (a) Gleiter, R.; Merger, R.; Chavez, J.; Oeser, T.; Irngartinger, H.; Pritzkow, H.; Nuber, B. Eur. J. Org. Chem. 1999, 2841-2843. (b) Haberhauer, G.; Gleiter, R.; Irngartinger, H.; Oeser, T.; Rominger, F. J. Chem. Soc., Perkin Trans. 2 1999, 2093-2097. (c) Gleiter, R.; Haberhauer, G.; Irngartinger, H.; Oeser, T.; Rominger, F. Organometallics 1999, 18, 3615-3622. (d) Stahr, H.; Gleiter, R.; Haberhauer, G.; Irngartinger, H.; Oeser, A. Chem. Ber. 1997, 130, 1807-1811. (e) Gleiter, R.; Merger, R. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH: Weinheim, 1995; pp 285-319. (f) Gleiter, R.; Scha¨fer, W.; Flatow, A. J. Org. Chem. 1984, 49, 372-374. (35) (a) Scott, L. T.; Cooney, M. J. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH: Weinheim, 1995; pp 321-351. (b) Scott, L. T.; Cooney, M. J.; Otte, C.; Puls, C.; Haumann, T.; Boese, R.; Carroll, R. J.; Smith, A. B., III; de Meijere, A. J. Am. Chem. Soc. 1994, 116, 10275-10283. (36) Matsuoka, T.; Negi, T.; Otsubo, T.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. Jpn. 1972, 45, 1825-1833. (37) (a) Toda, F.; Ando, T.; Kataoka, M.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1971, 44, 1914-1916. (b) Kataoka, M.; Ando, T.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1971, 44, 1909-1914. (c) Kataoka, M.; Ando, T.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1971, 44, 177-184. (d) Ando, T.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1971, 44, 172-177. (e) Toda, F.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1961, 34, 874-786. (f) Toda, F.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1961, 34, 862-874. (g) Toda, F.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1960, 33, 223-230.

assembling highly conjugated systems for materials applications has increased dramatically during the past decade,5,6 an understanding of electronic and structural characteristics based on angular distortions is essential. In the course of investigating the effects of cross conjugation in expanded radialenes such as 3 and 4,38 it became necessary to assess the influence of ring strain on these and related molecules.39 Since the degree of conjugation could be enhanced, diminished, or otherwise altered in strained structures, our ability to assess electronic changes as a function of molecular structure was viewed as fundamental toward tuning and tailoring the bulk properties of these new organic materials. In particular, the effects of strain for conjugated macrocycles containing a butadiyne segment,21-24 such as in 4, held specific appeal due to the rich history of this moiety in the formation of polydiacetylenes.40-42 Reported herein are the syntheses and characterization of a surprisingly stable series of crossconjugated, enyne macrocycles, 8a-e,38,43 also deemed cyclic expanded dendralenes,44 and the acyclic model compound 12. The extent and effects of ring strain in these macrocycles have been thoroughly examined by 13C NMR, Raman, and UV-vis spectroscopies, as well as X-ray crystallography for all six molecules 8a-e and 12. Results and Discussion Syntheses. The cyclic expanded dendralenes 8a-e were constructed as outlined in Scheme 1. Diynes 6a-e were coupled with vinyl triflate 545 (2.2 equiv) in the presence of Et2NH, CuI, (38) Eisler, S.; Tykwinski, R. R. Angew. Chem., Int. Ed. 1999, 38, 19401943. (39) Other examples of expanded radialenes: (a) Schreiber, M.; Tykwinski, R. R.; Diederich, F.; Spreiter, R.; Gubler, U.; Bosshard, C.; Poberaj, I.; Gu¨nter, P.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Jonas, U.; Ringsdorf, H. AdV. Mater. 1997, 9, 339-343. (b) Anthony, J.; Boldi, A. M.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Seiler, P.; Knobler, C. B.; Diederich, F. HelV. Chim. Acta 1995, 78, 797-817. (c) Boldi, A. M.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 468-471. (40) Baughman, R. H.; Yee, K. C. J. Polym. Sci. Macromol. ReV. 1978, 13, 219-239. (41) Polydiacetylenes; Bloor, D., Chance, R. R., Eds.; Martinus Nijhoff: Dordrecht, 1985. (42) Wegner, G. Makromol. Chem., Suppl. 1984, 6, 347-357. Wegner, G. Makromol. Chem. 1972, 154, 35-48. (43) For a preliminary communication of 8c-e, see: Tykwinski, R. R. Chem. Commun. 1999, 905-906. (44) Hopf, H. Angew. Chem., Int. Ed. Engl. 1984, 23, 948-960. (45) Stang, P. J.; Fisk, T. E. Synthesis 1979, 438-440.

Characteristics of Enyne Macrocycles

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Scheme 2. Attempted Synthesis of Macrocycle 9

Scheme 3. Synthesis of Model Compound 12

and Pd(PPh3)4.46 The coupling process proceeds rapidly at room temperature and works equally well in either DMF or THF solvent. Overall yields of tethered enediynes 7a-e were consistently modest with the mono-coupled product always isolated as the major byproduct. Protodesilylation of 7a-e with K2CO3 in MeOH/THF (1:1) for 2-3 h at room temperature affords the terminal alkynes. The desilylation proceeds in essentially quantitative yield with no evidence of byproduct formation. Following aqueous workup, these products were immediately carried on to the next step due to limited stability of desilylated product. Oxidative acetylenic coupling of the respective terminal alkynes of 7a-e was accomplished in CH2Cl2 (ca. 0.001 M) with CuI and tetramethylethylenediamine (TMEDA).47 Under these relatively dilute conditions, the cyclization reactions were generally complete in 2-5 h as monitored by TLC. Macrocycles 8a-e were isolated as white solids in modest yields by column chromatography. Always apparent by TLC analysis were the acyclic oligomers that result from intermolecular oxidative coupling of the diyne precursors. Unfortunately, this intermolecular coupling reaction always competes effectively with intramolecular cyclization, regardless of the reaction concentration or conditions. In an attempt to complete the series of strained cyclic dendralenes, the synthesis of highly strained 9 was also attempted (Scheme 2). In contrast to cycles 8a-e, however, 3,3-dimethyl-1,4-pentadiyne (10)35b was used due to its ready availability and greater stability when compared to 1,4-pentadiyne. Coupling of 10 with vinyl triflate 5 gave tetrayne 11 as a white solid in acceptable yield of 25%. The trimethylsilyl groups of 11 were easily removed with K2CO3 to give the terminal diyne, which was then subjected to the oxidative cyclization conditions in CH2Cl2 (0.001 M) until TLC analysis no longer showed the presence of starting material. Clearly evident by TLC are numerous compounds of increasing polarity that likely correspond to linear oligomers formed from the oxidative coupling process. 1H NMR and MS analysis of the resultant products, however, provided no evidence for the formation of the desired cycle 9. Synthesis of the acyclic model compound 12 (Scheme 3) began with the coupling of 1-hexyne to vinyl triflate 5, affording enediyne 13 in good yield as a light yellow oil that slowly decomposes at room temperature when exposed to air. Protodesilylation of 13 (K2CO3 in MeOH/THF) gave the terminal acetylene that was directly subjected to oxidative coupling protocol to afford dimer 12 in 49% yield as a white solid.

Physical Characteristics and X-ray Crystal Structures. Acyclic tetrayne 12 has a sharp melting point at 71 °C and pure macrocycles 8a-e are all thermally stable solids with melting or decomposition points above 100 °C. Cycles 8c and 8e show defined melting points of ca. 130 and 138 °C, respectively, whereas 8b shows no melting point and decomposes at just under 110 °C. The most strained cycle, 8a, shows darkening at 110 °C, finally blackening at 146 °C. Heating a single or microcrystalline sample of macrocycle 8d to 135 °C gave an increasingly orange colored crystal that eventually melted reproducibly at 168 °C.48 The solid-state structures of the dendralenes 8a-e and 12 were confirmed by X-ray crystallographic analysis. X-ray quality crystals were obtained by the diffusion of MeOH into CHCl3 solutions (8b-e and 12) or CH2Cl2 (8a) at 4 °C. ORTEP drawings of all structures are shown in Figure 1, whereas Tables 1-3 list crystallographic details and selected bond lengths and angles, respectively.49 Comparison of these six structures progressing from highly strained 8a to relatively strain-free 8e and acyclic 12 provides an understanding of the geometric changes as a result of the increasing ring strain. Empirically, macrocycles 8a, 8d, and 8e are approximately symmetrical (C2, Cs, and C2, respectively), whereas the 13- and 14-carbon cycles 8b and 8c are distorted considerably from C2 or Cs symmetry. This distortion in 8c was initially thought to derive simply from crystal packing effects. Semiempirical structural analysis, however, provides a geometry for 8c that is similar to the X-ray structure in which one “half” of the molecule is considerably more strained than the other.50 This suggests the symmetry distortion in solid 8c results from the inability of the four methylene linking groups to orient in a manner that equally disperses the ring strain, rather than from packing forces. The distortion of 8b in the solid state is even

(46) Zhao, Y.; Tykwinski, R. R. J. Am. Chem. Soc. 1999, 121, 458459. (47) Hay, A. S. J. Org. Chem. 1962, 27, 3320-3321.

(48) Powder samples of 8d show darkening at ca. 115 °C, and a reproducible melting point at ca. 155 °C. Continued heating gives rapid decomposition to a black solid at 165 °C. Differential scanning calorimetery of microcrystalline 8d showed a distinct melting point at 160 °C, followed almost immediately by decomposition or polymerization. These observations suggested a topochemical reaction/polymerization involving the diacetylene moiety to a polydiacetylene (PDA) product. Analysis of the crystal packing of 8d, however, shows the closest distance between atoms of the diacetylene groups to be greater than 7 Å, rendering PDA formation via a 1,4-addition pattern unlikely (see: Enkelmann, V. AdV. Polym. Sci. 1984, 63, 91-136). Analysis of the orange-colored material from the above experiments by 1H NMR and MS shows predominantly the macrocycle 8d. To date, the source of the discoloration remains unresolved, and investigations of the thermochemistry of 8d are currently ongoing. (49) The numbering scheme shown for structure 8c in Figure 1 has been changed from that used in ref 43 to facilitate comparisons to the other structures. (50) AM1 forcefield, MacSpartan Plus 1.1.9, Wavefunction, Inc., 1998.

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Eisler et al.

a

b

c

d

e

f

Figure 1. ORTEP drawings (20% probability level) of (a) 8a, (b) 8b, (c) 8c, (d) 8d, (e) 8e, and (f) 12.

more dramatic than that of 8c. In the case of 8b, calculations also predict a distortion from symmetry, albeit to a much smaller

extent than observed experimentally in the crystal structure.50 Raman spectroscopic analyses (vide infra) of both 8b and 8c

Characteristics of Enyne Macrocycles

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Table 1. Crystal, Intensity Collection, and Refinement Data for 8a-e and 12 lattice formula formula wt space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z temp/K radiation (λ, Å) F(calcd)/g cm-3 µ(Mo KR)/mm-1 2θ limit, deg no. of datab no. of obs datac no. of parameters Rd Rwe GOFf

8a

8b

8c

8d

8e

12

monoclinic C18H16 232.31 P21/na 6.5313(5) 26.949(2) 15.7798(14)

triclinic C19H18 246.33 P1h (No. 2) 8.3222(13) 8.3227(13) 11.0132(17) 97.810(3) 96.413(3) 91.112(3) 750.5(2) 2 193 0.71073 1.090 0.061 52.80 3058 2111 176 0.0624 0.1760 0.969

monoclinic C20H20 260.36 P21/c (No. 14) 9.2119(8) 12.5305(12) 14.5747(10)

monoclinic C21H22 274.39 P21/c (No. 14) 8.4095(9) 8.3657(9) 24.095(3)

107.744(7)

92.601(2)

1602.3(2) 4 213 0.71073 1.079 0.061 50 2814 1399 185 0.0762 0.1977 1.010

1693.4(3) 4 193 0.71073 1.076 0.060 51.5 3225 1478 194 0.0474 0.1106 0.837

triclinic C23H26 302.44 P1h (No. 2) 12.2451(13) 12.6784(11) 13.5270(12) 66.965(6) 89.087(8) 80.200(8) 1901.4(3) 4 193 0.71073 1.056 0.059 50 6334 3139 423 0.0672 0.1536 1.012

triclinic C24H30 318.48 P1h (No. 2) 7.2155(11) 7.6090(11) 10.0757(15) 106.604(3) 103.335(3) 100.228(3) 497.93(13) 1 193 0.71073 1.062 0.059 52.80 2023 1400 111 0.0479 0.1325 0.985

97.878(2) 2751.2(4) 8 193 0.71073 1.122 0.063 51.4 5217 2076 333 0.0526 0.1229 0.818

a A nonstandard setting of P2 /c (No. 14). b Unique data with F 2 g -3σ(F 2). c [F 2 g 2σ(F 2)]. d R ) ∑||F | - |F ||/∑|F |. e Rw ) ∑w(F 2 1 o o o o o c o o Fc2)2/ ∑w(Fo4)1/2. f [∑w(Fo2 - Fc2)2/(n - p)]1/2 (n ) number of data; p ) number of parameters varied; w ) [σ2(Fo2) + (aoP)2 + a1P]-1, where P ) [Max(Fo2,0)2 + 2Fc2]/3) and the coefficients ao and a1 are suggested by the least-squares program; 8a, ao ) 0.0388, a1 ) 0; 8b, ao ) 0.1156, a1 ) 0; 8c, ao ) 0.0764, a1 ) 0.5537; 8d, ao ) 0.0374, a1 ) 0; 8e, ao ) 0.0478, a1 ) 0.1698; 12, ao ) 0.0726, a1 ) 0.

Table 2. Selected Bond Lengths (Å) for 8a-e and 12 C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(4)-C(8) C(8)-C(9) C(9)-C(10) C(10)-C(11) C(11)-C(12) C(12)-C(13) C(12)-C(16) C(16)-C(17) C(17)-C(18) a

8aa

8ab

8b

8c

8d

8ea

8eb

12

1.477(4) 1.197(4) 1.442(4) 1.341(3) 1.448(4) 1.195(4) 1.370(4) 1.209(3) 1.442(4) 1.339(3) 1.446(4) 1.198(4) 1.467(4)

1.458(4) 1.185(3) 1.454(4) 1.345(3) 1.442(4) 1.205(3) 1.381(4) 1.206(3) 1.436(4) 1.343(3) 1.449(4) 1.188(4) 1.482(4)

1.465(3) 1.205(3) 1.440(3) 1.350(3) 1.435(3) 1.208(3) 1.372(3) 1.209(3) 1.438(3) 1.347(3) 1.454(3) 1.192(3) 1.463(3)

1.461(5) 1.180(5) 1.455(5) 1.336(4) 1.439(5) 1.196(4) 1.378(5) 1.207(5) 1.433(5) 1.340(5) 1.443(5) 1.188(5) 1.466(6)

1.472(3) 1.193(3) 1.443(3) 1.346(3) 1.431(3) 1.198(3) 1.374(3) 1.202(3) 1.436(3) 1.342(3) 1.441(3) 1.197(3) 1.469(3)

1.472(5) 1.193(4) 1.444(5) 1.348(4) 1.441(4) 1.201(4) 1.382(5) 1.194(4) 1.440(4) 1.339(4) 1.448(5) 1.194(4) 1.461(5)

1.476(5) 1.185(4) 1.443(5) 1.351(4) 1.441(4) 1.200(4) 1.374(5) 1.198(4) 1.439(5) 1.350(4) 1.446(5) 1.188(5) 1.465(5)

1.468(2) 1.195(2) 1.444(2) 1.357(2) 1.4434(19) 1.2038(18) 1.377(3)c

Data for molecule A of a crystallographically independent pair. b Data for molecule B of a crystallographically independent pair. c C(9)-C(9′).

Table 3. Selected Bond Angles (deg) for 8a-e and 12 C(1)-C(2)-C(3) C(2)-C(3)-C(4) C(3)-C(4)-C(5) C(6)-C(5)-C(7) C(3)-C(4)-C(8) C(4)-C(8)-C(9) C(8)-C(9)-C(10) C(9)-C(10)-C(11) C(10)-C(11)-C(12) C(11)-C(12)-C(13) C(14)-C(13)-C(15) C(11)-C(12)-C(16) C(12)-C(16)-C(17) C(16)-C(17)-C(18)

8aa

8ab

8b

8c

8d

8ea

8eb

12

177.1(3) 167.9(3) 126.4(3) 116.6(2) 108.7(2) 157.1(3) 159.8(3) 158.7(3) 156.7(3) 125.7(3) 116.6(2) 108.4(2) 168.5(3) 175.1(3)

175.4(3) 169.1(3) 125.8(3) 116.4(2) 108.5(2) 155.3(3) 159.5(3) 160.1(3) 156.6(3) 126.0(3) 116.3(2) 108.9(2) 168.6(3) 174.0(4)

173.07(19) 170.66(19) 125.15(17) 116.65(17) 110.49(16) 163.7(2) 164.6(2) 163.4(2) 156.88(19) 126.12(18) 117.19(17) 107.45(16) 161.6(2) 172.0(2)

176.6(4) 171.0(4) 125.2(3) 116.6(3) 110.9(3) 166.5(4) 169.0(4) 168.2(4) 161.7(4) 125.4(4) 117.1(3) 108.4(3) 165.4(4) 170.5(4)

179.5(2) 170.5(2) 124.7(2) 116.77(18) 111.47(19) 168.0(2) 168.8(2) 169.4(2) 166.4(2) 124.2(2) 115.7(2) 111.2(2) 170.4(2) 178.7(3)

174.2(4) 171.9(4) 124.2(3) 116.1(3) 112.4(3) 172.7(4) 177.3(4) 176.6(4) 173.5(4) 123.0(3) 115.9(3) 112.7(3) 174.0(4) 173.7(4)

176.1(4) 173.0(4) 123.4(3) 116.7(3) 113.2(3) 172.2(4) 176.4(4) 176.8(4) 172.5(4) 122.8(3) 115.9(3) 113.6(3) 173.0(4) 177.0(4)

179.19(15) 177.02(15) 123.34(13) 116.20(13) 114.77(12) 177.21(15) 179.1(2)c

a Data for molecule A of a crystallographically independent pair. b Data for molecule B of a crystallographically independent pair. c C(8)C(9)-C(9′).

display characteristics consistent with dissymmetric structures in solution. No evidence of symmetry distortion, however, was observed in solution on the time scale of 1H or 13C NMR analysis.

The relative planarity of each macrocycle in its preferred conformation is certain to influence its electronic behavior. The planarity of the conjugated portion of cycles 8a-e and acyclic 12 in the solid state varies considerably and is not associated

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Table 4. Selected 13C NMR Shift Data (in ppm) for 8a-e and 12a

8a 8b 8c 8d 8e 12 a

C(2), C(17) CH2sCtCs

C(3), C(16) CH2sCtCs

C(4), C(12) CtCsC(dC)sCtC

C(5), C(13) CtCsC(dC)sCtC

C(8), C(11) sCtCsCtCs

C(9), C(10) sCtCsCtCs

90.3 95.7 94.6 93.7 93.8 93.1

81.2 81.5 80.7 79.5 79.1 76.4

103.5 103.2 102.8 102.2 101.8 100.9

145.0 145.6 146.9 148.5 151.3 156.0

105.1 98.9 90.4 88.4 81.7 79.5

86.5 82.5 80.0 78.2 75.9 75.4

All spectra obtained in CDCl3.

directly with decreasing ring size as might be expected. For each compound, the extent of twisting can be estimated by a dihedral angle between the two “arms” of the conjugated segment linked by the butadiyne moiety. Two planes, defined by C(3)-C(4)-C(5)-C(8) and C(11)-C(12)-C(13)-(16), respectively, provide this estimate of planarity. The conjugated portion of acyclic model compound 12 is completely planar, with a twist angle that is necessarily 180° based on solid state symmetry and the transoid orientation of the two alkylidene units about the butadiyne linker. For the most conformationally flexible macrocycle 8e, with two crystallographically independent molecules A and B, the twist angle is greatest for molecule A at 22.2(2)° and somewhat smaller for molecule B at 12.7(3)°. The twist is approximately equal for 8c and 8d at 3.36(19)° and 2.82(13)°, respectively. The planarity of 8b is quite distorted as a result of the dissymmetry of the molecule, with an angle of 8.22(16)°. Despite the considerably increased ring strain of 8a, the twist angles at 7.0(2)° and 6.2(2)° for the two crystallographically independent molecules A and B, respectively, are greater than that of either 8c or 8d. Thus, increased twist and bond strain in the conjugated portion of 8a is tolerated in order to accommodate the bond angles demanded by the two methylene groups. Overall, the range of bond angles about the sp3-hybridized carbons of the methylene groups of 8a-e and 12 span a maximum of 119° to a minimum of 109° (both in 8c). While the correlation between methylene bond angles and ring size is not linear, the general trend observed in these angles also relays the increased ring strain as evidenced by the mean values of for 8a, 8b, 8c, 8d, and 8e of 115.9, 112.4, 115.0, 114.2, and 113.0°, respectively. By way of comparison, angles about the methylene carbons 12 are found, on average, to be 113.6°. Structural analysis of the six molecules 8a-e and 12 also shows that, in contrast to bond angles, the CsC, CdC, and CtC bond lengths differ very little between the different structures. No discernible trends emerge that would correlate bond length to strain, and the bond lengths found in acyclic 12 are well within the same range as those of 8a-e. NMR Spectroscopic Properties. The 1H and 13C NMR spectra of macrocycles 8a-e and acyclic 12 are completely consistent with their structures. None of the macrocycles 8a-e shows any evidence of multiple conformers on the time scale of NMR measurements; the observed number of carbon resonances and line shapes clearly suggest the expected degeneracy for carbons related by C2 or Cs symmetry. Unfortunately, however, the resonances for five of the six sp2 and sp carbons fall within a similar range between 75 and 105 ppm. This fact made the empirical assignment of each resonance impossible.51 Furthermore, due to the extensive multibond H-C coupling for these highly unsaturated molecules, shift assignments based on coupled, one-dimensional 13C NMR techniques (51) Using an empirical analysis, the 13C NMR shifts of C(4)/C(12) for 8c-e were incorrectly reported in ref 43.

were also problematic as the values of 3J, 4J, and 5J were similar in magnitude.52 The two-dimensional gradient heteronuclear multiple bond coherence (gHMBC) experiments for 8a-e and 12, however, were sufficient for assignment of all resonances for the conjugated carbon skeletons, as summarized in Table 4. In all cases, a similar strategy was used for interpretation of each gHMBC spectra. The alkylidene methyl protons of C(6)/C(15) and C(7)/C(14) for each molecule showed strong two- and threebond (2JC-H and 3JC-H) correlations to the endo- and exocyclic alkylidene carbons C(4)/C(12) and C(5)/C(13), respectively (see X-ray structures in Figure 1 for numbering scheme). The exocyclic vinylidene carbons C(5)/C(13) are known to be significantly deshielded,53 and thus C(4)/C(12) and C(5)/C(13) were assignable. Weaker four-bond correlations were observed between the alkylidene methyl protons and C(3)/C(16) and C(8)/ C(11). In some cases, weak five-bond couplings to C(9)/C(10) and/or C(2)/C(17) were also observable. The 2JC-H and 3JC-H correlations between the propargylic methylene protons of C(1) and acetylenic carbons C(2) and C(3) identified these carbons. On the basis of the stronger 4J coupling between C(3)/C(16) and the alkylidene methyl protons, C(2)/C(17) and C(3)/C(16) were thus assigned. The assignments of C(2)/C(17) and C(3)/ C(16) were corroborated for 8b, 8d, and 12 by three-bond correlations between C(2) and the homopropargylic methylene protons of C(19) of 8b, C(21) of 8d, and C(10) of 12. The 5JC-H coupling between the C(1) methylene protons and C(8) allowed the assignment of C(8)/C(11), leaving the assignment of C(9)/ C(10) to the last 13C NMR resonance. The more shielded endocyclic alkylidene carbon C(4)/C(12) resonances are found over a rather narrow shift range between 100.9 and 103.5 ppm. For acyclic 12, these carbons resonate at 100.9 ppm, and they are slightly deshielded to 101.8 ppm for the largest macrocycle 8e. As the ring size is decreased from 17 to 12 upon moving from 8e to 8a, respectively, a deshielding is observed for these carbons to give values of 102.2 (8d), 102.8 (8c), 103.2 (8b), and 103.5 (8a). These shifts are consistent with the expected rehybridization at C(4)/C(12), as bond angle contraction for C(3)-C(4)-C(8) and C(11)-C(12)-C(16) imparts greater p-character to the σ-bonds of the ring and, consequently, more s-character to the olefin.33,39b,54 Conversely, the chemical shifts of the exocyclic vinylidene C(5)/C(13) carbons are shifted significantly upfield as a function of strain, from the value observed for acyclic 12 at δ 156.0 to 151.3 (8e), 148.5 (8d), 146.9 (8c), 145.6 (8b), and 145.0 (8a). A strong correlation is found between the crystallographicdetermined bond angles and 13C NMR shifts of the sp and sp2 carbons. The angles R were calculated as an average of the X-ray angles at 13C NMR degenerate carbons (Table 5). As shown in (52) Kalinowski, H. O.; Berger, S.; Braun, S. Carbon-13 NMR Spectroscopy; John Wiley & Sons: Chichester, 1988. (53) Neidlein, R.; Winter, M. Synthesis 1998, 1362-1366. (54) Wong, H. N. C.; Sondheimer, F. Tetrahedron 1981, 37, W99W109.

Characteristics of Enyne Macrocycles

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Table 5. Averaged Angles R about sp- and sp2-Hybridized Carbon in 8a-e and 12

a

R values (°) compd

C(2)/ C(17)

C(3)/ C(16)

C(4)/ C(12)a

C(8)/ C(11)

C(9)/ C(10)

8ab 8b 8c 8d 8eb 12

175.4 172.5 173.6 179.1 175.3 179.2

168.5 166.1 168.2 170.5 173.0 177.0

108.6 109.0 109.7 111.3 113.0 114.8

156.4 160.3 164.1 167.2 172.8 177.2

159.5 164 168.7 169.1 176.8 179.1

b

a C(3)-C(4)-C(8)/C(11)-C(12)-C(16). b Averaged over both crystallographically independent molecules A and B of the X-ray structure.

Figure 2, the relationship between the alkylidene bond angle R at C(3)-C(4)-C(8)/C(11)-C(12)-C(16) and the 13C NMR resonances observed for both C(4)/C(12) and C(5)/C(13) is linear. Changes in the relative bond order of the acetylenic moieties C(2)-C(3) and C(8)-C(9) (as well as C(10)-C(11) and C(16)C(17), respectively) of 8a-e and 12 are also easily monitored by their 13C NMR resonances.55 All acetylenic carbons experience consistent deshielding as ring strain is increased. A similar trend has been reported by Gleiter and co-workers for nonconjugated, cyclic diynes.56 In particular, the carbons of the butadiyne carbons C(8)/C(11) and C(9)/C(10) experience significant deshielding as a result of increased ring strain. In the case of C(8)/C(11), the carbons are deshielded by 25 ppm upon going from acyclic 12 (79.5 ppm) to 8a (105.1 ppm) as bond angles R are decreased from 177 to 156°, respectively. Concurrently, the C(9)/C(10) carbons are deshielded by 11 ppm, from δ 75.4 ppm in 12 to δ 86.5 ppm in 8a, while the corresponding bond angles decrease from 179° in acyclic 12 to 159.6° in 8a. The relationship between acetylenic bond angles and 13C NMR shifts is nearly linear for all acetylenic carbons with the exception of C(2)/C(17).55 The only deviation from the above analysis is the anomalous 13C NMR chemical shift observed for acetylenic carbons C(2)/ C(17) of the most strained cycle 8a (Table 4). This carbon resonance for 8a is shielded by 5.4 ppm relative to 8b, and by 3.5 ppm vs 8e, which has the most similar bond angle with R ) 175.3°. The origin of this shielding remains unclear. Based on the work of Gleiter and co-workers,56,57 however, two possible influences emerge. The transannular distance between C(2) and C(17) is 3.08 Å for 8a. This distance is at the limit of that required for through-space orbital overlap between these carbons and could potentially alter the chemical shift. The transannular distance between C(2) and C(17) for all other cycles is too large for such interactions to occur (e.g., it is already 4.04 Å in 8b). Alternatively, shielding of C(2)/C(17) acetylenic carbons might result from through-bond interactions between the ethano bridge and the in-plane π-system. Based on photoelectron spectroscopic experiments, similar σ-π electronic interactions have been reported in cycles such as 1,5-cyclooctadiyne, although no concurrent change in 13C NMR chemical shift was reported for this molecule.56 Electronic Absorption Characteristics. Two distinct π-electron systems must be considered in the electronic absorption (55) See Supporting Information for correlations of 13C NMR chemical shift data with bond angle R about acetylenic carbons C(2)/C(17), C(3)/ C(16), C(8)/C(11), and C(9)/C(10). (56) Gleiter, R.; Kratz, D.; Scha¨fer, W.; Schehlmann, V. J. Am. Chem. Soc. 1991, 113, 9258-9264. (57) Gleiter, R.; Merger, R.; Irngartinger, H. J. Am. Chem. Soc. 1992, 114, 8927-8932.

Figure 2. Plot of 13C NMR chemical shift for (a) C(4)/C(12) and (b) C(5)/C(13) versus angle R C(3)-C(4)-C(8)/C(11)-C(12)-C(16).

Figure 3. Schematic representation of the (a) in-plane and (b) outof-plane p-orbital systems found in cyclic alkynes 8a-e and acyclic 12.

analysis of cyclic dendralenes 8a-e and acyclic 12. One system is composed of the acetylenic π-orbitals directed, more or less, in the molecular plane of each macrocycle (Figure 3a). The inplane system of the butadiyne group is indicative of this system, and its electronic absorption should be observable at approximately 260 nm. Contributions to the in-plane butadiyne π-electron system via homoconjugation with the “pendant” alkynes are possible if the alkylidene bond angles were sufficiently constricted. Conversely, the out-of-plane π-electron system includes the eight carbons of the linearly conjugated ene-yne-yne-ene segment (Figure 3b). Interactions via cross conjugation46 from the two pendant acetylenic units could influence the electronic absorption(s) of this π-electron system, which are expected in the lower energy region of each electronic absorption spectrum. The UV absorption maxima of 8a-e and acyclic analogue 12 are listed in Table 6, and the spectra for 8a-c and 8c-e, 12 are shown in Figure 4a and b, respectively. Empirically apparent are the similar energies of lower energy absorptions of 12 and those of macrocycles 8c-e at ca. 290, 309, and 329 nm (Figure 4b), absorptions that arise from the out-of-plane π-electron system of each compound. The major difference in the spectrum of acyclic 12 versus cycles 8c-e is the considerable broadening and lower -values of the low-energy absorptions for 12. The latter is attributed to inhomogeneous broadening as a result of the rotational freedom of the alkylidene units about the butadiyne moiety of 12. This gives both cisiod and transoid absorption

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Eisler et al.

Table 6. Selected UV Absorption Data (in nm) for 8a-e and 12a absorption maxima, λmax () 8a 259 (30 000) 262 (30 500) 8b 260 (30 900) 8c 258 (25 000) 8d 259 (26 000) 8e 260 (29 000) 12 260 (21 700) a

276 (21 100) 295 (13 900) 313 (29 900) 333 (26 000) 275 (12 900) 275 (10 700) 275 (11 300) 276 (8 400) 271 (19 100)

292 (19 900) 290 (18 600) 291 (19 300) 290 (15 700) 286 (13 900)

311 (31 100) 309 (31 500) 309 (32 400) 309 (26 500) 307 (15 800)

331 (27 800) 329 (29 500) 329 (30 500) 328 (24 500) 327 (12 500)

All spectra obtained in CHCl3. λmax in nm and  in L M-1 cm-1.

evidently derives from increased ring strain rather than homoconjugation (vide infra), which would arise from the in-plane π-system. It is not unreasonable to expect that the significant distortion from linearity in 8a and 8b slightly diminishes the overlap of the out-of-plane π-orbitals of these molecules. This has the effect of raising the energy of the HOMO to a slightly greater extent than that of the LUMO, resulting in a slightly lower energy transition.11 The red-shifted λmax observed for 8a and 8b contrasts results reported for o,o′-bridged polymethylene ether derivatives of diphenylacetylenes, 14, the most structurally related system to be rigorously studied, to the best of our knowledge.11,37a,e,f In this case, a decrease in ring size for 14 from a planar, rigid structure (n ) 5) to a strained, planar derivative (n ) 3) afforded a hypsochromic shift of 12 nm in the lowest energy absorption. Modeling predicted that the p-orbitals contributing to conjugation, including the phenyl rings, are held parallel regardless of the magnitude of strain; thus, the observed blue shift was attributed entirely to ring strain.37a,f Conversely, ring contraction in the p,p′-bridged system 15 gave a bathochromic shift in λmax from 342.5 nm in the strain-free, planar structure (n ) 18) to 345.5 nm in a highly strained and nonplanar structure (n ) 13).37a,b In contrast to cycles 14, however, the conjugated p-orbitals in cycles 15 are no longer parallel as the two benzene groups deviate from coplanarity in more strained derivatives. It was suggested that this nonparallel alignment was responsible for the red shift in λmax for the more strained derivatives of 15. It is thus clear that the electronic characteristics in cross-conjugated systems cannot be effectively predicted on the basis of established studies of linearly conjugated systems.

Figure 4. Electronic absorption spectra ( [L M-1 cm-1]) in CHCl3 comparing (a) cyclic alkynes 8a-c and (b) cyclic alkynes 8c-e and acyclic 12.

peaks in which the trans conformer absorbs at a slightly lower energy.38 Peak broadening and lower -values are also observed for macrocycle 8e relative to 8d and 8c. This is likely due to the more flexible 17-carbon cyclic framework of 8e, which affords a lower overall molar absorptivity based on increased deviations from planarity allowed by the long -(CH2)7- tether. Macrocycles 8d and 8c show the greatest -values, and this correlates well with the almost negligible twist angles (2.82(13)° and 3.36(19)°, respectively) observed in the solid state for the conjugated portions of these molecules. Molar absorptivitiy for the most strained systems 8b and 8a is diminished relative to 8c as twisting increases to accommodate the shorter -(CH2)3and -(CH2)2- tethers in these conformationally restricted macrocycles. Whereas the lower energy electronic absorptions for 8c-e show little variation in energies, further increasing ring strain on going from 8c to 8b to 8a results in a consistent bathochromic shift for all three low-energy absorptions. For example, λmax of 8c at 329 nm shifts to 331 nm in 8b and to 333 nm in 8a (Figure 4a). As it is the overlap of the eight conjugated out-of-plane carbon sp and sp2 π-orbitals of the ene-yne-yne-ene segment that yields these absorption bands, this bathochromic shift

It has been shown for a series of [6]-, [4]-, and [3]radialenes that increased rigidity and enforced planarity as a function of ring contraction gives a decreasing absorption energy in a trend similar to that observed for 8a-e,58 although the overall (58) Hopf, H.; Maas, G. Angew. Chem., Int. Ed. Engl. 1992, 31, 931954.

Characteristics of Enyne Macrocycles conjugation length is lower in the radialenes than in 8a-e. Slight changes in the electronic absorption spectra as a function of ring size have also been reported for expanded radialenes, which contain a similar ene-yne-yne-ene moiety as 8a-e.39b The degree of strain, planarity, and steric interactions in these systems, however, is unknown and makes meaningful comparisons impossible. The UV spectrum of the smallest dendralene 8a shows evidence of homoconjugation, which affects the absorption of the in-plane butadiyne π-system at ca. 260 nm.35b,59 Specifically, cycles 8b-e and acyclic 12 all display this high-energy absorbance at 258-260 nm with increased broadening as the ring size is contracted. The spectrum of the highly strained cycle 8a shows a substantially broadened peak at 263 nm, in addition to the shoulder absorption at 259 nm. This bathochromic shift in 8a relative to 8b-e and 12 is most reasonably ascribed to a through-space interaction of the in-plane π-orbital system of 8a as a result of significantly deceased alkylidene bond angles. These results mirror those found previously for cycle 16.35b As a result of homoconjugation, strained cycle 16 (λmax ) 266 nm) shows bathochromic shifts of 11 nm versus acyclic 17 (λmax ) 255 nm) and 10 nm versus the cyclic diyne 18 (λmax ) 256 nm). Also supporting the premise of homoconjugation in 16 is the 7 nm bathochromic shift of λmax for 16 versus unstrained percycline 19 (λmax ) 259 nm). The interior bond angles of the saturated carbons of 16 at 103.8° are only 5° smaller than the alkylidene angles of 8a at 108.6°.35b Raman Spectroscopic Properties. Raman and resonance Raman characterization of electronic structures have been reported for cycloalkynes.1,18 None of these studies, however, has addressed the electronic structure of macrocyclic molecules with extended or cross conjugation. Either or both of these attributes should afford significantly different electronic structures relative to less functionalized and linearly conjugated examples. In previous investigations, correlations have been observed between lower vibrational frequencies and smaller Cs CtC bond angles, and these observations were attributed to rehybridization of the central carbon atom from sp toward sp2. The rehybridization results in a lower force constant of the Ct C stretch and altered coupling between the CtC and CsC stretches as the CsCtC bond angle varies. These observations are variations of Badger’s rule,60,61 which empirically relates diatomic stretching frequencies to the electronic nature of the bond. These preceding results all suggest that Raman spectroscopy should be a useful probe of electronic structure in our novel cross-conjugated enyne macrocycles. Raman spectra of cross-conjugated enynes 8a-e and 12 are shown for the 900-1700 cm-1 region in Figure 5 and for the 1400-2300 cm-1 region in Figure 6. The 900-1700 cm-1 spectral region contains contributions from CsC and CdC stretches and C-H deformations, whereas the 2100-2300 cm-1 spectral region contains contributions only from CtC stretches. The Raman spectra are of good quality, although the spectrum of 8b exhibits a slightly lower signal-to-noise ratio than the other spectra due to fluorescence in this sample. This fluorescence, however, decayed after several minutes in the laser beam, indicating that it likely originates from the presence of a small amount of fluorescent impurity rather than from the dendralene.62 Examination of the spectra in Figures 5 and 6 reveals some important trends. The spectra all exhibit the greatest intensity (59) Armitage, J. B.; Whiting, M. C. J. Chem. Soc. 1952, 2005-2010. (60) Badger, R. M. J. Chem. Phys. 1934, 2, 128-131. (61) Gordy, W. J. Chem. Phys. 1946, 14, 305-320. (62) No impurity could be detected by TLC or 1H NMR analysis.

J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6925

Figure 5. Raman spectra of cross-conjugated macrocycles 8a-e and acyclic 12 in the 900-1700 cm-1 spectral region, displaced vertically for clarity. Typical accumulation times were 15 min; n is number of methylene groups.

in the CtC stretch(es), followed closely by the CdC stretch(es); both the CsC stretches and CsH deformations, probably arising from the methylene groups, are much less intense. These intensities are to be expected, as the double and triple bonds are much more polarizable than the methylene groups. Close inspection of the CdC and CtC stretches reveals frequency shifts and mode splittings as a function of the increased ring strain upon moving from unstrained 12 to strained 8a, consistent with the altered electronic structure exhibited in the crystal structure, and both 13C NMR and electronic absorption spectra. In molecules 8b and 8c, with three and four methylene groups, respectively, the CdC and CtC modes are significantly split, indicating either an inhomogeneity in the sample (e.g., different conformers) or nonequivalence in two or more similar bonds in the molecule. As the X-ray structures of 8b and 8c clearly show nonsymmetrical geometry in the solid state for both molecules, it is apparent that in solution there are indeed two nonequivalent sets of sp and sp2 bonds. As discussed above, this premise has been supported by a semiempirical analysis that also predict nonsymmetrical geometries for 8b and 8c. For the CdC stretching frequencies (Figure 5), a clear shift to lower frequency is observed, indicating a lower bond order for the alkylidene group as ring strain is decreased. The shift in olefinic stretching frequencies nicely complements the trend observed in the 13C NMR shift data, relating the double bond order as a function of ring strain. Indeed, a plot (not shown) of the CdC stretching frequencies for 8a-e (frequencies averaged for 8b and 8c) versus the 13C NMR shifts of C(4)/C(12) and C(5)/C(13) affords a linear relationship in both cases, with

6926 J. Am. Chem. Soc., Vol. 122, No. 29, 2000

Eisler et al. the force constant, but also the vibrational character of the normal mode. Conclusions We have presented the synthesis and comprehensive analyses for a series of cyclic dendralenes that relate structural and spectroscopic attributes as a function of ring strain. The degree of bond angle deformation that is clearly depicted in the respective X-ray structures for each molecule can be directly correlated to trends in Raman and 13C NMR spectroscopic data and directly reflect the varying electronic structure. Despite significant deviations from optimal bond angles for cycles 8a and 8b, ring strain exerts only a small influence on the electronic absorption spectra of these compounds, giving a slight bathochromic shift in λmax values. In the most strained cycle 8a, dramatic angle contraction affords changes in the higher energy region of the UV spectrum that can be attributed to the throughspace π-orbital interactions of homoconjugation. A number of the correlations between strain and spectroscopic properties described herein show empirical trends similar to those reported for other less conjugated or nonconjugated systems. Other strainbased correlations for 8a-e, such as the bathochromic shift in electronic absorptions of strained dendralenes 8a,b, however, run contrary to previously reported studies and demonstrate the need for thorough analysis of new and structurally unprecedented systems. The structural relationships between these six homologous molecules 8a-e and 12 provide a solid basis for a better understanding of the fundamental properties of these and future cross-conjugated macrocycles. Experimental Section

Figure 6. Same as Figure 5, but in the 1400-2300 cm-1 spectral region.

correlation coefficients of 0.975 and 0.999, respectively.63 Likewise, a plot (not shown) of the alkylidene bond angles R for C(3)-C(4)-C(8)/C(11)-C(12)-C(16) versus olefinic stretching frequency is also linear, with a correlation coefficient of 0.995.64 Analogous trends in bond order as a function of ring strain have been reported in radialenes58 and have been well studied for alicyclic ketones and alkylidene cycloalkanes.65 In contrast, however, the current study extends the usefulness of Raman analysis for the study of macrocycles with increased conjugation and potential materials applications. The CtC stretching frequency (or frequencies) of the strained dendralenes monotonically increases upon going from the 8a to acyclic 12, indicating a greater bond order as ring strain is decreased. The trend observed for CtC stretching frequency (or frequencies) mirrors those observed for 13C NMR shifts of the acetylenic carbons and averaged bond angles R, although with more substantial deviations from linearity than described above in the analysis of CdC data.1,18,66 Smaller deviations have been observed in cycloalkynes1 and are attributed in the present case to altered vibrational mixing, which will change not only (63) See Supporting Information for correlations of (a) olefinic Raman frequencies with 13C NMR chemical shift data for carbons C(4)/C(12) and C(5)/C(13), and b) olefinic Raman frequencies with bond angle R C(3)C(4)-C(8)/C(11)-C(12)-C(16). (64) See Supporting Information for correlations of acetylenic Raman frequencies with bond angle R about carbons C(8)/C(11) and C(9)/C(10). (65) Bellamy, L. J. AdVances in Infrared Group Frequencies; Methuen: London, 1968. (66) Detert, H.; Meier, H. Liebigs Ann./Recl. 1997, 1565-1570.

General. Reagents were purchased reagent grade from commercial suppliers and used without further purification. THF was distilled from sodium/benzophenone ketyl. 1-Hexyne and 6b-d were purchased from Aldrich and 6a from GFS chemicals. Compounds 5,45 10,35b and 6e67 were prepared as described. Anhydrous MgSO4 was used as the drying agent after aqueous workup. Evaporation and concentration in vacuo was done at H2O-aspirator pressure. All reactions were performed in standard, dry glassware under an inert atmosphere of N2. A positive pressure of N2 was essential to the success of all Pd-catalyzed reactions. Degassing of solvents was accomplished by vigorously bubbling N2 through the solution for at least 45 min. Column chromatography employed silica gel-60 (230-400 mesh) from General Intermediates of Canada. Thin-layer chromatography (TLC) used aluminum sheets covered with silica gel-60 F254 from Macherey-Nagel; visualization was done by UV light or KMnO4 stain. Melting points were determined with a Gallenkamp apparatus and are uncorrected. UV/vis spectra were recorded using a Pharmacia Biotech Ultrospec 3000 at room temperature; λmax is reported in nm ( in L M-1 cm-1). IR spectra (cm-1) were obtained on a Nicolet Magna-IR 750 (neat) or Nic-Plan IR Microscope (solids). 1H and 13C NMR studies were done with Varian Gemini-300 or -500 and Bruker AM-300 instruments, at room temperature in CDCl3; solvent peaks (7.24 ppm for 1H and 77.0 ppm for 13C) are used as reference. gHMBC spectra were recorded using a Varian Gemini-500 instrument at room temperature in CDCl3. EI MS (m/z) were obtained with a Kratos MS50 instrument. Pro Fit Version 5.1.2 from QuantumSoft (Zu¨rich, Switzerland) was used for all data analyses. Raman. Room-temperature Raman spectra of the cross-conjugated enynes 8a-e and 12 were obtained with 1-mL solutions of the sample in CCl4. Raman scattering was excited by spherically focusing the 514.5-nm laser line from an Ar ion laser (Coherent, Santa Clara, CA) onto a spinning 5-mm NMR tube containing the sample solution in a 135° backscattering geometry. The laser power was typically 200 mW. Multichannel detection of the resonance Raman scattering was ac(67) Brandsma, L. PreparatiVe Acetylenic Chemistry, 2nd ed.; Elsevier: Amsterdam, 1988.

Characteristics of Enyne Macrocycles complished with a liquid nitrogen-cooled CCD detector (Princeton Instruments, Trenton, NJ) connected to the first half of a double monochromator (Spex Industries, Metuchen, NJ). Rejection of the Rayleigh line was accomplished by a holographic filter (Kaiser Optical Systems, Ann Arbor, MI). Spectral slit widths were 5-7 cm-1. Frequency calibration was performed by measuring Raman scattering of solvents of known frequencies (acetone, acetonitrile, and ethanol) and by measuring atomic emission lines from Kr and Ne lamps. Reported frequencies are accurate to (2 cm-1. The resonance Raman spectra were analyzed by using a 486DX2-66V computer (Gateway Computers, North Sioux City, SD). A CCl4 solvent spectrum was subtracted from all solution spectra. The baselines were leveled by subtracting multiple joined line segments from the spectrum. General Procedure for the formation of Tethered Enediynes. The appropriate diyne 6a-e was added to a degassed solution of vinyl triflate 5 (2-2.1 equiv) in THF (20 mL). Pd(PPh3)4 (ca. 0.05 equiv) and Et2NH (0.5-2 mL) were sequentially added, the solution was stirred for 5 min, CuI (ca. 0.15 equiv.) was added, and the solution was stirred until TLC analysis no longer showed progression toward the product (1-3 h.). In a number of cases, reaction progress ceased with a mixture of monocoupled product, product, and triflate all present in TLC analysis. Additional palladium catalyst or extended reaction times never effected additional product formation. Following completion of the reaction, ether and H2O were added, the organic phase was separated, washed with saturated aqueous NH4Cl (2 × 50 mL), and dried, and the solvent was removed in vacuo. Flash column chromatography gave the desired products. 3,10-Bis(trimethylsilylethynyl)-2,11-dimethyldodeca-2,10-diene4,8-diyne (7a). Compound 7a (0.034 g, 45%) was obtained as a white solid according to the general procedure from the reaction of 1,5hexadiyne (0.050 mL of 4 M solution in pentane, 0.20 mmol), vinyl triflate 5 (0.13 g, 0.43 mmol), Pd(PPh3)4 (12 mg, 0.010 mmol), Et2NH (1 mL), and CuI (4 mg, 0.02 mmol) in THF (5 mL). Rf ) 0.45 (5:1 hexanes/CH2Cl2). Mp ) 80-82 °C. IR (solid): 2956, 2149, 1601, 1438 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.64 (s, 4H), 2.03 (s, 6H), 2.02 (s, 6H), 0.23 (s, 18H). 13C NMR (CDCl3, 75.5 MHz): δ 154.7, 102.2, 101.4, 95.6, 90.5, 78.1, 22.6 (2×), 20.0, 0.1. EI HRMS: calcd for C24H34Si2 378.2199, found 378.2205. 3,11-Bis(trimethylsilylethynyl)-2,12-dimethyltrideca-2,11-diene4,9-diyne (7b). Compound 7b (0.059 g, 61%) was obtained as a white solid according to the general procedure from the reaction of 1,6heptadiyne (0.024 g, 0.246 mmol), vinyl triflate 5 (0.147 g, 0.488 mmol), Pd(PPh3)4 (17 mg, 0.015 mmol), Et2NH (0.5 mL), and CuI (7 mg, 0.04 mmol) in THF (6 mL). Rf ) 0.15 (20:1 hexanes/CH2Cl2). Mp ) 67-69 °C. IR (CHCl3 cast): 2935, 2145, 1595, 1428 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.53 (t, J ) 7.1 Hz, 4H), 2.03 (s, 6H), 2.01 (s, 6H), 1.83 (p, J ) 7.1 Hz, 2H), 0.23 (s, 18H). 13C NMR (CDCl3, 75.5 MHz): δ 154.1, 102.4, 101.5, 95.5, 91.3, 77.8, 28.0, 22.6 (2×), 18.8, 0.1. EI HRMS: calcd for C25H36Si2 392.2356, found 392.2357. 3,12-Bis(trimethylsilylethynyl)-2,13-dimethyltetradeca-2,12-diene4,10-diyne (7c). Compound 7c (0.135 g, 39%) was obtained as a white solid according to the general procedure from the reaction of 1,7octadiyne (0.090 g, 0.85 mmol), vinyl triflate 5 (0.600 g, 2.00 mmol), Pd(PPh3)4 (49 mg, 0.042 mmol), Et2NH (2 mL), and CuI (25 mg, 0.13 mmol) in DMF (25 mL). Rf ) 0.45 (4:1 hexanes/CH2Cl2). Mp ) 7879 °C. IR (solid): 2958, 2229, 2150, 1459 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.36 (t, J ) 6.0 Hz, 4H), 1.97 (s, 6H), 1.95 (s, 6H), 1.66 (m, 4H), 0.17 (s, 18H). 13C NMR (CDCl3, 125 MHz): δ 153.9, 102.4, 101.6, 95.4, 91.9, 76.8, 27.9, 22.6 (2×), 19.1, 0.1. EI HRMS: calcd for C26H38Si2 406.2512, found 406.2505. 3,13-Bis(trimethylsilylethynyl)-2,14-dimethylpentadeca-2,13-diene-4,11-diyne (7d). Compound 7d (0.137 g, 36%) was obtained as a light yellow oil according to the general procedure from the reaction of 1,8-nonadiyne (0.108 g, 0.900 mmol), vinyl triflate 5 (0.600 g, 2.00 mmol), Pd(PPh3)4 (150 mg, 0.130 mmol), Et2NH (2 mL), and CuI (50 mg, 0.26 mmol) in DMF (30 mL). Rf ) 0.2 (18:1 hexanes/CH2Cl2). IR (CHCl3 cast): 2959, 2148, 1433 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.34 (m, 4H), 1.97 (s, 6H), 1.95 (s, 6H), 1.55 (m, 6H), 0.17 (s, 18H). 13C NMR (CDCl , 125 MHz): δ 153.9, 102.5, 101.6, 95.4, 92.2, 76.8, 3 28.3, 28.2, 22.6, 22.5, 19.5, 0.1. EI HRMS: calcd for C27H40Si2 420.2669, found 420.2668.

J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6927 3,15-Bis(trimethylsilylethynyl)-2,16-dimethylheptadeca-2,15-diene-4,13-diyne (7e). Compound 7e (0.158 g, 26%) was obtained as a light yellow oil according to the general procedure from the reaction of 1,10-undecadiyne (0.200 g, 1.35 mmol), vinyl triflate 5 (0.900 g, 3.00 mmol), Pd(PPh3)4 (50 mg, 0.043 mmol), Et2NH (3 mL), and CuI (25 mg, 0.13 mmol) in DMF (30 mL). Rf ) 0.45 (4:1 hexanes/CH2Cl2). IR (CHCl3 cast): 2933, 2149, 1433 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.32 (t, J ) 6.9 Hz, 4H), 1.98 (s, 6H), 1.96 (s, 6H), 1.53 (m, 4H), 1.40 (m, 6H), 0.17 (s, 18H). 13C NMR (CDCl3, 75.5 MHz): δ 153.6, 102.5, 101.7, 93.5, 92.4, 76.6, 28.8, 28.7, 28.6, 22.5 (2×), 19.5, 0.1. ΕΙ HRMS: calcd for C29H44Si2 448.2982, found 448.2972. 3,9-Bis(trimethylsilylethynyl)-2,6,6,10-tetramethylundeca-2,9-diene-4,7-diyne (11). Compound 11 (0.090 g, 25%) was obtained as a white solid according to the general procedure from the reaction of dimethyl pentadiyne 10 (0.215 g, 0.907 mmol), vinyl triflate 5 (0.509 g, 1.69 mmol), Pd(PPh3)4 (52 mg, 0.045 mmol), Et2NH (1.0 mL), and CuI (15 mg, 0.078 mmol) in THF (10 mL). Rf ) 0.40 (5:1 hexanes/ CH2Cl2). Mp ) 104-106 °C. IR (CH2Cl2 cast): 2960, 2249, 2153, 1439 cm-1. 1H NMR (CDCl3, 300 MHz): δ 1.98 (s, 6H), 1.96 (s, 6H), 1.56 (s, 6H), 0.18 (s, 18H). 13C NMR (CDCl3, 75.5 MHz): δ 155.1, 102.1, 101.3, 95.7, 95.4, 76.4, 31.3, 27.2, 22.7, 22.6, 0.1. ΕΙ HRMS: calcd for C25H36 Si2 392.2356, found 392.2354. 3-Trimethylsilylethynyl-2-methylnon-2-en-4-yne (13). Compound 13 (0.103 g, 62%) was obtained as a light yellow oil according to the general procedure from the reaction of excess 1-hexyne (0.302 g, 3.67 mmol), vinyl triflate 5 (0.214 g, 0.714 mmol), Pd(PPh3)4 (24 mg, 0.021 mmol), Et2NH (1.0 mL), and CuI (11 mg, 0.058 mmol) in THF (5.0 mL). Rf ) 0.59 (5:1 hexanes/CH2Cl2). IR (CH2Cl2 cast): 2932, 2149, 1432 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.32 (t, J ) 6.9 Hz, 2H), 1.98 (s, 3H), 1.96 (s, 3H), 1.45 (m, 4H), 0.90 (t, J ) 7.2 Hz, 3H), 0.17 (s, 9H). 13C NMR (C6D6, 75.5 MHz): δ 152.6, 103.4, 102.8, 95.2, 92.6, 77.9, 30.8, 22.2, 22.1, 22.0, 19.2, 13.4, 0.1. EI HRMS: calcd for C15H24Si 232.1647, found 232.1638. General Procedure for Oxidative Coupling. A mixture of the appropriate enediyne 7a-e or 13 and K2CO3 (ca. 10 mg) in wet THF/ MeOH (1:1, 15-20 mL) was stirred at room temperature until TLC analysis showed complete conversion to the desilylated derivative (0.5-2 h). Ether and saturated aqueous NH4Cl were added, and the organic phase separated, washed with saturated aqueous NH4Cl (2 × 50 mL), dried, reduced to ca. 1 mL, and added to CH2Cl2 to give a solution of 1-3 mM. Catalyst [CuI (50 mg, 0.26 mmol) and TMEDA (75 mg, 0.65 mmol) in CH2Cl2 (2 mL), stirred until homogeneous] was added and the mixture stirred at room temperature under air until TLC analysis no longer showed the starting material (ca. 1-5 h). The solution was concentrated in vacuo to ca. 5 mL, ether and H2O were added, the organic phase was separated, washed with saturated aqueous NH4Cl (2 × 50 mL), and dried, and the solvent was removed in vacuo. Flash column chromatography and/or precipitation from MeOH gave the desired product. 5,12-Diisopropylidenecyclododeca-1,3,6,10-tetrayne (8a). Compound 8a (0.0122 g, 32%) was obtained as a white solid according to the general oxidative coupling procedure using 7a (0.0622 g, 0.164 mmol), CuI (60 mg, 0.32 mmol), and TMEDA (0.1 mL) in CH2Cl2 (60 mL). Rf ) 0.55 (2:1 hexanes/CH2Cl2). Sample discolors at 110 °C, decomposes at 146 °C. IR (CH2Cl2 cast): 2907, 2149, 1622, 1340 cm-1. 1H NMR (CDCl , 300 MHz): δ 2.65 (s, 4H), 1.98 (s, 6H), 1.90 (s, 3 6H). 13C NMR (CDCl3, 125 MHz): δ 145.0, 105.1, 103.5, 90.3, 86.5, 81.2, 23.4, 22.1, 19.8. UV/vis (CHCl3): λ () 333 (26 000), 313 (29 900), 295 (13 900), 276 (21 100), 263 (30 500), 259 (30 000) nm. EI HRMS: calcd for C18H16 232.1252, found 232.1253. 5,13-Diisopropylidenecyclotrideca-1,3,6,11-tetrayne (8b). Compound 8b (0.0074 g, 20%) was obtained as a white solid according to the general oxidative coupling procedure using 7b (0.0593 g 0.151 mmol), CuI (62 mg, 0.32 mmol), and TMEDA (0.1 mL) in CH2Cl2 (60 mL). Rf ) 0.34 (5:1 hexanes/CH2Cl2). Sample discolors at 100 °C, decomposes at 109-111 °C. IR (CHCl3 cast): 2904, 2145, 1617, 1346 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.45 (t, J ) 6.2 Hz, 4H), 2.00 (p, J ) 6.2 Hz, 2H), 1.90 (s, 6H), 1.85 (s, 6H). 13 C NMR (CDCl3, 125 MHz): δ 145.6, 103.2, 98.9, 95.7, 82.5, 81.5, 28.0, 23.1, 22.2, 19.8. UV/vis (CHCl3): λ () 331 (27 800), 311 (31 100), 292 (19 900), 275

6928 J. Am. Chem. Soc., Vol. 122, No. 29, 2000 (12 900), 259 (30 900) nm. EI HRMS: calcd for C19H18 246.1409, found 246.1408. 5,14-Diisopropylidenecyclotetradeca-1,3,6,12-tetrayne (8c). Compound 8c (0.0062 g, 34%) was obtained as a white solid according to the general oxidative coupling procedure using 7c (0.0285 g, 0.0701 mmol), CuI (28 mg, 0.15 mmol), and TMEDA (0.5 mL) in CH2Cl2 (30 mL). Rf ) 0.34 (5:1 hexanes/CH2Cl2). Mp ) 129-130 °C. IR (solid): 2905, 2214, 2192, 2118, 1618, 1334 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.35 (m, 4H), 1.91 (s, 6H), 1.89 (s, 6H), 1.74 (AA′BB′, 4H). 13C NMR (CDCl3, 125 MHz): δ 146.9, 102.8, 94.6, 90.4, 80.7, 80.0, 29.2, 22.9, 22.2, 19.9. UV/vis (CHCl3): λ () 329 (29 500), 309 (31 500), 290 (18 600), 275 (10 700), 258 (25 000) nm. EI HRMS: calcd for C20H20 260.1565, found 260.1564. 5,15-Diisopropylidenecyclopentadeca-1,3,6,13-tetrayne (8d). Compound 8d (0.024 g, 34%) was obtained as a white solid according to the general oxidative coupling procedure using 7d (0.110 g, 0.260 mmol), CuI (90 mg, 0.47 mmol), and TMEDA (0.15 mL) in CH2Cl2 (60 mL). Rf ) 0.25 (9:1 hexanes/CH2Cl2). Solid turns orange at 135 °C and then melts 168 °C. IR (solid): 2905, 2215, 2130, 1617, 1349 cm-1. 1H NMR (CDCl3, 300 MHz) δ 2.42 (t, J ) 5.6 Hz, 4H), 1.92 (s, 6H), 1.89 (s, 6H), 1.70 (m, 2H), 1.54 (m, 4H). 13C NMR (CDCl3, 125 MHz): δ 148.5, 102.2, 93.7, 88.4, 79.5, 78.2, 29.8, 28.9, 22.9, 22.2, 19.9. UV/vis (CHCl3): λ () 329 (30 500), 309 (32 400), 291 (19 300), 275 (11 300), 259 (26 000) nm. EI HRMS: calcd for C21H22 274.1722, found 274.1726. 5,17-Diisopropylidenecycloheptadeca-1,3,6,15-tetrayne (8e). Compound 8e (0.006 g, 22%) was obtained as a white solid according to the general oxidative coupling procedure using 7e (0.040 g, 0.089 mmol), CuI (30 mg, 0.16 mmol), and TMEDA (0.1 mL) in CH2Cl2 (30 mL). Rf ) 0.4 (4:1 hexanes/CH2Cl2). Mp: 136-138 °C. IR (solid): 2931, 2227, 2133, 1604 cm-1. 1H NMR (CDCl3, 300 MHz):

Eisler et al. δ 2.35 (m, 4H), 1.96 (s, 6H), 1.95 (s, 6H), 1.55 (m, 8H), 1.29 (m, 2H). 13C NMR (CDCl , 125 MHz): δ 151.3, 101.8, 93.8, 81.7, 79.1, 75.9, 3 30.3, 29.6, 28.9, 22.6, 22.5, 19.6. UV/vis (CHCl3): λ () 328 (24 500), 309 (26 500), 290 (15 700), 276 (8 400), 260 (29 000) nm. EI HRMS: calcd for C23H26 302.2035, found 302.2035. 7,11-Diisopropylideneoctadeca-5,8,10,13-tetrayne (12). Compound 12 (0.0346 g, 49%) was obtained as a white solid according to the general oxidative coupling procedure using 13 (0.103 g, 0.444 mmol), CuI (42 mg, 0.22 mmol), and TMEDA (1 mL) in CH2Cl2 (30 mL). Rf ) 0.32 (2:1 hexanes/CH2Cl2). Mp ) 71-72 °C. IR (CH2Cl2 cast): 2932, 2221, 2136, 1590, 1361 cm-1. 1H NMR (CDCl3, 300 MHz): δ 2.33 (t, J ) 6.9 Hz, 4H), 2.01 (s, 6H), 1.99 (s, 6H), 1.47 (m, 8H), 0.90 (t, J ) 7.2 Hz, 6H). 13C NMR (CDCl3, 125 MHz): δ 156.0, 100.9, 93.1, 79.5, 76.4, 75.4, 30.8, 22.7, 22.0, 21.1, 19.1, 13.6. UV/vis (CHCl3): λ () 327 (12 500), 307 (15 800), 286 (13 900), 271 (19 100), 260 (21 700) nm. EI HRMS: calcd for C24H30 318.2347, found 318.2349.

Acknowledgment. This work was supported by a GenScience Endowment from the University of Alberta (to R.R.T.) and by NSERC of Canada (R.R.T. and G.R.L.). We thank Dr. Tom Nakashima for extensive help with the 13C NMR studies, and Ms. Navjot Chahal for making a number of the starting materials. Supporting Information Available: X-ray structural data, and gHMBC NMR spectra for 8a-e and 12, and correlations of 13C NMR chemical shift data, bond angles R, and Raman frequencies (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. 1H, 13C,

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